Sharing of operational experience on plate distillation columns

Preface: Distillation technology is widely used in the production of various chemicals, and distillation towers are also one of the common devices in chemical plants. During the production process, the operation of the distillation tower is particularly important; it is used not only for the purification of the final product but also for the purification of raw materials, as well as for the recovery of solvents and waste materials. In addition, in the production of certain fine chemicals, it is directly involved in the reaction process. Therefore, to help everyone become more familiar with the operation of distillation towers, Xiao Qi will provide a detailed introduction to the process of operating such towers! 01 Startup of the distillation column. Startup is a very important step in production. The objectives are to shorten the startup time, reduce costs, prevent potential accidents, and produce qualified products as quickly as possible. General steps for driving: a. Establish reasonable driving procedures, a schedule, and necessary preventive measures ； Prepare the necessary raw materials and supplies of water, electricity, and steam ； Assign the necessary staff and complete the relevant training, etc. b. At this point, the structure of the tower must meet the design requirements; it should be clean, free of any solid debris or blockages, and all substances that should not be present there must be removed. For example, the oxygen and moisture content within the tower must be within specified limits ； The pumps and instruments are functioning properly after calibration ； The safety measures have been adjusted. c. Apply pressure and release pressure to the tower to reach the normal operating pressure. d. Heat and cool the tower to bring it close to the operating temperature. e. Add raw materials to the tower. f. The heat source for activating the tower top condenser, as well as the reboiler and various heaters; the cold source for various coolers. g. Gradually adjust the operating conditions and parameters of the tower, so that the load on the tower and the quality of the product can reach the normal operating values as quickly as possible, thereby enabling the tower to operate under normal conditions. 02 Shutdown of the distillation tower: After the plant has been in operation for a certain period of time, various problems arise with the equipment and instruments. Continuing production no longer meets economic and practical requirements in terms of production capacity and raw material consumption, and it also poses a potential risk of accidents; therefore, the plant needs to be shut down for maintenance. General steps for shutting down: a. Determine a load-reduction plan to gradually lower the load on the tower, simultaneously reducing the amount of heat applied and the coolant used, until operation is completely halted. If there is direct steam in the column (such as in the main fractionation column of a catalytic cracking unit), to prevent liquid leakage from the tray and to obtain more high-quality product, the amount of direct steam can be increased appropriately when reducing the throughput. b. Stop feeding. c、Liquid remaining in the emission tower. d. Carry out operations such as reducing or increasing the pressure in the tower, lowering or raising the temperature, and cleaning or flushing with an inert gas, to bring the tower to near normal temperature and pressure, in preparation for opening the access hole to allow atmospheric entry and to facilitate maintenance work. The specific preparatory work required must be determined based on the particular conditions of the tower, adapting to local circumstances. During normal operation of the tower, gas rises through the holes in the tray plates, while the liquid flows across the surface of the plates, over the overflow weir, and into the downcomer to reach the next tray plate. When the reactor has just started operating, steam tends to rise through the downcomers and the vapor channels on the tray surfaces, while the liquid tends to leak out through the holes in the tray rather than flowing across the tray surfaces into the downcomers. It only gradually returns to normal flow conditions when the gas-liquid flow rate is appropriate enough to establish a liquid seal in the downcomer. Three criteria for establishing a liquid seal are: (1) The flow velocity of the gas through the pores in the tray must be high enough to prevent liquid from leaking out of these pores, allowing the liquid to flow across the tray and over the overflow weir to reach the downcomer. (2) Initially, the gas velocity as it flows through the downcomer must be low enough so that the liquid can fall after passing over the overflow weir and flow through the downcomer. (3) The downcomer must be sealed by liquid, that is, the height of the liquid layer in the downcomer must be greater than the bottom clearance of the downcomer. Full reflux operation: Full reflux operation is frequently employed during the start-up of distillation columns. When there is a temporary interruption in feed supply to the column, full reflux operation is also used to maintain optimal operating conditions. Additionally, full reflux operation serves as a method for removing moisture from within the column. Starting up with full reflux is generally simple and effective, as the column is not affected by the operations of upstream equipment, allowing sufficient time to adjust its operation. Under full reflux, it is easy to establish a concentration profile in the column, enabling the product composition to reach the specified values; this approach also helps to save on the amount of liquid used and reduces the quantity of defective products. During full reflux operation, either the feed liquid or the products that are qualified or unqualified from the tower can be used; the conditions established in such a tower are relatively similar to those during normal operation, and it is easy to adjust things to obtain qualified products once normal feeding and operation begin. For high-purity columns with a large reflux ratio, starting up with full reflux is highly attractive. The literature recommends this starting method for ethylene and propylene distillation columns, as it takes a relatively long time for such columns to go from the start-up stage to stable operation, and full reflux results in conditions within the column that are closer to those under normal operation. For columns with a low reflux ratio or those that are easy to operate, it is not necessary to use the full-reflux startup method. The start-up method using full reflux is not very suitable for the following two situations, or certain measures need to be taken: ① Unwanted reactions may occur during prolonged full reflux operation, especially in the region of higher temperatures at the bottom of the tower (unless appropriate materials can be selected that do not undergo such reactions under full reflux conditions, then full reflux should be avoided in such cases). ②The materials contain trace amounts of hazardous substances, such as trace vinyl acetylene in the butadiene distillation column, and trace methylene acetylene and methyl acetylene in the propylene distillation column. They do not cause problems during normal operation, but during prolonged full reflux operation, if there are leaks in the valves of the overhead distillate pipeline, this is equivalent to batch distillation; over time, these harmful substances gradually accumulate in the tower, leading to explosions or other accidents. In the full reflux operation of the butadiene distillation column, accidents caused by explosions due to the accumulation of vinyl acetylene have been reported; therefore, the feed material should not contain such trace substances. For example, methylacetylene and dimethylacetylene in the propylene-propane stream must be removed through hydrogenation before a full reflux operation can be used for startup; otherwise, it should be avoided. 03 What preparations are needed before starting up a distillation tower? Check whether water, electricity, gas (air, nitrogen), and steam (water vapor) meet the process requirements ； Is there a spare transmission unit available? ； Are the equipment, instruments, and safety facilities complete and in good working condition? ； All valves must be in the closed position ； A water condenser (cooler) needs to be fed with a small amount of water for pre-cooling, while the heating kettle requires a small amount of steam for pre-heating ； The oxygen content within the equipment should meet the requirements for feeding ； Ensure good coordination between upstream and downstream units (or positions), paying special attention to the supply of raw materials as well as the storage and transportation of products; inform the analysis laboratory to prepare for sample collection and analysis. What are the specific requirements for the pressure testing of equipment model 04? Generally, water is used as the testing medium. Unless there are any special requirements, the following procedure can be followed: for equipment with a working pressure of less than 5 kg/cm², the test pressure should be 1.5 times the working pressure (except for cast iron equipment) ； For those with a working pressure of 5 kg/cm2 or higher, the test pressure is 1.25 times the working pressure ； The working pressure is less than 2 kg/cm2, while the test pressure is 2 kg/cm2. For vessels operating at atmospheric pressure, a water leak test alone is sufficient. During the test, the pressure of the equipment should be increased gradually. When a leak or other defect is detected, the pressure must be reduced to normal levels; it is not allowed to perform maintenance on the equipment while it is under pressure to avoid accidents. After the maintenance is complete, boost the voltage again and conduct another inspection. During the pressure testing process, the following points should be followed: In strength testing, after reaching the specified pressure, it is necessary to wait for 5 minutes, then reduce the pressure to the operating level. A hammer weighing 0.5–1.5 kilograms should be used to strike an area 150 millimeters away from both sides of the weld, in order to check for any leaks or deformation. The allowable pressure drop within one hour is: when the volume of the equipment is 1 cubic meter or less, the allowable drop is less than 1% ； When the volume of solvent in the equipment is between 1 and 3 cubic meters, the allowable decrease is less than 0.5% ； When the volume of the equipment is 3 m3 or more, the allowable drop is less than 0.2%. Normal temperature water is generally used as the medium for strength testing, and the water should be injected at the lowest point of the equipment so that the gas inside can be vented from the highest point of the equipment. 05 Why is it necessary to purge equipment and pipelines for new equipment or equipment that has undergone major repairs? The purpose of purging is to remove dust, welding rods, iron shavings, and other debris that remain in the equipment or pipelines during installation. The purging gas is generally compressed air. The inspection method for cleaning is to use a white gauze to check at the end of the cleaning process; if no black spots are present, the cleaning is considered successful. The direction of purging should be from the highest point of the equipment downward. 06 Why is a gas-tightness test required for equipment before it is put into operation? Are there any specific requirements? The purpose of the airtightness test is to ensure, through testing, that the equipment is airtight and leak-free; if any leaks exist, they can be rectified before operation begins. In this way, leaks of toxic, flammable, and explosive materials are prevented, ensuring continuous and normal production. The requirements for the airtightness test are as follows: The medium used for the airtightness test is most commonly air ； However, for equipment where no air is allowed after testing, nitrogen should be used. To reduce the amount of nitrogen used, an air test can be conducted first; once it is successful, nitrogen can then be used to replace the air. Test the oxygen content in the exhaust air; production can only begin once it meets the specified standards. The pressure gauge used for testing should have an appropriately higher accuracy to facilitate the detection of leaks. The pressure for the airtightness test must be 1.1 times the operating pressure. For equipment with a working pressure of 5 kg/cm2 or higher, the test pressure is 1.05 times the working pressure ； The pressure reduction value is generally 2 kg/cm2; special requirements apply to equipment operating at high temperatures, and these should be determined based on the process requirements and the equipment design drawings. The airtightness test of the system requires maintaining pressure for 24 hours ； For the airtightness test of single units, it is stipulated that the pressure must be maintained for 8 hours. It is best to use two pressure gauges and two thermometers during the experiment for measurement purposes, in order to provide a comparison. The passing requirement for the airtightness test is that the average leakage rate per hour must not exceed 0.25%. 07 Why is it necessary to replace the air with an inert gas before putting the equipment into operation? In the petrochemical industry, many of the materials that are separated are flammable and explosive. If the air inside the equipment (mainly oxygen) is not removed before operation, there is a risk of fire and explosion after feeding the materials in, so it is necessary to replace the air inside the equipment with an inert gas before putting it into use. The inert gas remaining after displacement can eliminate potential accident risks, as it does not undergo any chemical reaction with the material that has been separated. Inert gases can be gradually purged during startup. The commonly used inert gas is nitrogen. Before commissioning, the oxygen content in the gas within the equipment is generally required to be no more than 2% (by volume). 08 What are the disadvantages of excessive water accumulation in the distillation tower during startup? When a distillation tower used for separating water-immiscible organic compounds is started up, an excessive amount of water in the reactor can cause the temperature of the reactor bottom to drop; this phenomenon occurs in the distillation towers for butadiene, styrene, and phenol. Sometimes, even though the temperature in the reactor has risen, it can still drop due to the gradual return of moisture from the tray to the bottom of the reactor. This is because drainage is necessary to raise the temperature of the kettle to normal levels. Before starting up the extractive distillation column, if there is an excess of water inside the column and it is not removed in time, the addition of the extractant will result in a decrease in its concentration; therefore, water must also be absent from the column before startup. The reason for the low pot temperature can be considered from the principle of steam distillation. That is, at the bottom of the tower, water and the material form two immiscible phases. When heated, the boiling point of the mixture will be lower than that of any of its components. Due to this decrease in the boiling point of the mixture, the temperature at the bottom of the tower cannot reach the desired process parameters. 09 What preparations are needed before starting vacuum distillation? Each vacuum pump shall undergo individual testing, with the pumping capacity and vacuum level both meeting the requirements specified in the process specifications ； After the individual towers and equipment pass the single-unit trial run and airtightness tests, a vacuum level test is conducted ； The system is evacuated, ready for feeding and production. 10 What should be noted when starting up a distillation tower? Upon receiving the start-up command, immediately contact the relevant departments to proceed with starting up. Strictly adhere to the process procedures and operating guidelines, and strengthen routine inspections. Carefully adjusted. The feed rate should be steady; once a liquid level is observed in the bottom of the tower, the temperature should be increased slowly according to the specified rate until the process parameters are reached. As the tower pressure increases, the inert gas inside the equipment is gradually removed, and the cooling capacity of the top condenser is progressively increased. Reflux is initiated once the liquid level in the reflux tank reaches more than 1/2 of its capacity. When the liquid level in the reactor reaches 2/3, it is possible to decide whether to remove the liquid from the reactor or to reduce or even stop the feed rate to the tower, depending on the temperature of the reactor; however, the liquid level in the tower reactor must always be maintained between 1/2 and 2/3. Once the operation is stable, material analysis should be conducted. Unqualified materials can be removed in small quantities or sent back through the process entirely; once the analysis shows that the materials are qualified, production can proceed on a continuous basis. When driving, it is essential to adjust the valves and instruments frequently and slowly, and to use them in a proper manner. If any abnormality is detected, analyze the cause promptly and take decisive action. 11 What are the differences in starting up an extractive distillation column compared to a regular distillation column? During the startup of extractive distillation, the extractor is first circulated without adding any material, and the process parameters are increased in temperature. Only after a normal circulation has been established for the extractor within the standard process flow can materials be added. 12 Why must the temperature of the slurry increase slowly while driving? During charging to an empty column, there is no reflux liquid, so the trays in the distillation section operate in a dry-tray condition. Due to the lack of gas-liquid contact, the poorly volatile components in the gas phase can easily be carried into the distillation section. If the heating rate is too fast, a large amount of the less volatile components will be carried to the distillation section, where they are not easily displaced by the more volatile components. As a result, the quality of the product at the top of the tower cannot meet the required standards, leading to longer startup times. Once there is reflux liquid at the top of the tower and a liquid layer is formed on the tray, the heating rate can be increased appropriately. The heating rate of the vacuum distillation tower has a more significant impact on the success of startup. 13 What are the categories of distillation column shutdowns? It is divided into temporary parking and long-term parking: Temporary parking: Upon receiving the parking command, feeding to the tower, distillation at the top of the tower, and extraction from the bottom of the tower are immediately stopped, and full reflux operation is carried out. Appropriately reduce the cooling load at the top of the tower and the heating load at the bottom, keeping the entire tower in a state of heat and pressure retention. If the parking time is short, it can be handled according to the specific conditions of the tower: only the feed to the tower can be stopped, while the product taken from the top of the tower can continue to flow, in order not to affect the production in subsequent stages; however, the withdrawal of product from the bottom of the tower should be stopped. This operation disrupts the normal material balance and should not be used for extended periods, otherwise the product quality will decline. Long-term parking: After issuing the parking command, stop feeding material to the tower immediately; product extraction can continue. If the analysis results are unsatisfactory, extraction can be stopped, along with heating of the tower bottom and condensation at the tower top, after which the liquid in the bottom of the tower should be drained. For columns used to separate low-boiling-point materials, the drainage of the bottom liquid should be carried out slowly to prevent excessively low temperatures resulting from throttling, which could cause the material of the equipment to become brittle. After the discharge is complete, release the residual pressure inside the equipment and then replace it with nitrogen; maintenance can only be carried out once this is done successfully. If it is necessary to enter the equipment for maintenance, the nitrogen must be replaced with air; maintenance personnel are only allowed to enter when the oxygen content in the gas inside the equipment reaches 19% (by volume) or more.

I saw it in other materials; I hope it will be helpful to you

A book titled Questions and Answers on Distillation Operation Techniques is quite good

For problems with distillation towers, it is necessary to analyze each specific issue individually. For example, flooding. It is necessary to determine what causes flooding. Otherwise, it will have the opposite effect. For example, a high pressure difference in the tower could be caused by freezing blockage, flooding or submersion of the tower, or an excessive load.